Can osteoporosis be seen on x-rays?
If the bone loss is sufficient, it can become visible on routine X-rays. However X-rays are not very sensitive and do not indicate early osteoporosis. Studies have shown that more than 30% of bone mass must be lost before it appears clearly as osteoporosis on X-rays. Therefore, if osteoporosis does show up on a routine X-ray, it is already quite advanced and more difficult to treat successfully. One of the difficulties with osteoporosis is that often, the first indication of the disease is a fracture, clearly this is too late! We therefore need to measure indicators of both the current status of the skeleton and the ongoing remodelling process.

Bone Densitometry
The density of our bones is known to be the best indicator of their strength and hence their resistance to fracture. The density of our bones is generally measured at sites most at risk of suffering the effects of osteoporosis, i.e. the cancellous bone sites of the spine, hip and wrist.

Bone density is often referred to as bone mineral density (BMD). The term density describes the mass (weight, measured in grams (g)) of bone within a certain volume (measured in cubic centimetres (cm³)). Hence, the units of density are grams per cubic centimetre or g cm^-3. However, when most of the clinically available systems (e.g. DXA) undertake a bone densitometry scan, we measure the area of bone that has been interrogated but not the thickness. Hence, BMD is often measured in units of grams per square centimetre, (g cm^-2)).

Bone densitometry measures the status of our skeleton at a particular time. Changes in our bone density over time may only be assessed by repeat BMD measurements, at intervals of one or two years.

When not to measure BMD
Contraindications for spinal BMD include pregnancy (abdominal thickness, radiation dose), administration of oral contrast media and radioisotopes, spinal deformity and orthopaedic hardware (e.g. hip replacement). BMD results could be affected by metal objects such as belts, buttons, brassieres etc. and recent ingestion of calcium-containing tablets. The presence of osteomalacia and osteoarthritis will cause under- and over-estimation of BMD respectively. Previous fracture, severe scoliosis, small stature and obesity will also affect BMD.

Bone Markers
We may assess the status of the bone remodelling process by measuring chemical indicators of the amount of bone being removed and added, termed bone markers. A range of tests may be performed using either blood or urine samples.

Next Section - Bone Densitometry

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